1. Field of the Invention
The present invention relates generally to a linear motor, and more specifically to a permanent magnet type linear motor for use in a machine tool or the like.
2. Description of Relate Art
Prior to turning to the present invention, it would be preferable to describe, with reference to FIGS. 6(A) to 6(C) and FIGS. 7 and 8, a conventional linear motor which is pertinent to the present invention. FIG. 6(A) is a side elevation of a linear motor (generally denoted by reference number 8), and FIG. 6(8) is a cross section taken along a section line A-B of FIG. 6(A).
As shown FIG. 6(A), the linear motor 8 generally comprises a stator 10 and a movable body 12. The movable body 12 is positioned apart from the stator 10 leaving an air gap therebetween so as to be movable with respect thereto. The stator 10 is comprised of a plurality of permanent magnets 14, each of which is rectangular in this case and has a lengthwise axis in the direction normal to the drawing of FIG. 6(A). The magnets 14 are arms in an equally spaced manner on a plate-like iron base (support) 16. The polarities of the magnets 14 are such as to change alternately in the moving direction of the movable body 12 (viz., in the horizontal direction in the drawing). Each of the arrows attached to the permanent magnets 14 indicates the magnetized direction of the corresponding magnet. On the other hand, the moving body 12 comprises an armature core 18 of magnetic material and a plurality of armature coils 20 respectively wound around the teeth (or leg-like projections) 22 of the core 18.
The armature coils 20 are concentratedly wound around the teeth 22 of the core 18, respectively, and coupled so as to take the form of balanced three-phase circuit, wherein the three phases are respectively denoted by U, V, and W. The balanced three-phase circuit or connection per se is well known in the art, and thus the further description will be omitted for simplifying the instant disclosure.
In the linear motor 8 shown in FIG. 6(A), the eight permanent magnets 14 are arranged such as to oppose nine teeth 22 of the armature core 18. Arrows shown in FIG. 6(B) schematically indicate the winding directions of the armature coils 20 provided around the teeth 22. The nine coils 20 produce eight magnetic fields between the adjacent teeth. As is known in the art, when the phases of the currents flowing through the coils 20 are controlled, the moving body 12, which is held by a suitable support (not shot), moves linearly above the stator 10.
The number of the teeth 22 and the number of the permanent magnets 14 opposing them, are not limited to the above-mentioned ones. By way of example, FIG. 6(C) shown one example wherein the nine teeth 22 of the armature core 18 are arranged such as to face the six permanent magnets 14, in the case of which the coils 20 carried by the nine teeth 22 are supplied with the U-, V- and W-phase currents so as to generate six magnetic fields at the side of the magnets 14.
FIGS. 7(A) and 7(B) schematically illustrate the manner wherein the linear motor 8 of FIG. 6(A) is installed in a machine tool 30 (only part thereof is illustrated). FIG. 7(A) is a front elevation of the linear motor 8 together with the part of the machine tool 30 as viewed in the direction of the movement of the movable body 12. FIG. 7(B) is a schematic side elevation of the linear motor 8 and part of the stator 10 of FIG. 7(A) as viewed in the direction perpendicular to the movement of the movable body 12.
As shown in FIG. 7(A), the movable body 12 is fixed to the lower side of a table 32 which is provided with linear guides 34a and 34b extending in the direction of the movement of the movable body 12. The stator 10 has been mounted on a plate-like bottom member 36 of the machine tool 30. The machine tool 30 is further equipped with two side members 38a and 38b which stand vertically at the opposite ends of the bottom member 36. The machine tool 30 is still further equipped with two linear guides 40a and 40b on the tops of the side members 38a and 38b, respectively. The above-mentioned guides 34a and 34b, which are fired to the lower side of the table 32, are slidably mounted on the liner guides 40a and 40b when the movable body 12 has been assembled with the stator 10.
At the final stage of assembly of the linear motor 8 as shown in FIGS. 7(A) and 7(B), the table 32 accompanying the movable body 12 is lowered as shown by arrows 42a and 42b. Thus, the armature core 22 of magnetic material approaches the permanent magnets 14 with the result in occurrence of an extremely large amount of magnetic attraction force imparted on the magnetic core 18 as indicated by a broad open arrow 44. Accordingly, in order to precisely position the movable body 12 on the stator 10 against such a large magnetic force, it is inevitable to prepare a jig (typically rigid and bulk which is dedicated to the assembly itself. However, it is practically difficult to settle such jig on or in the vicinity of the machine tool 30 due to a limited working space. In the case where a suitable jig is unable to prepare for the assembly of the linear motor 8 FIG. 6(A)), the above-mention prior art has encountered the problem that it needs a fairly long time until completing the assembly of the linear motor 8, in addition to which it is not typically expected to precisely mount the movable body 12 onto the stator 10, leading to the fact that the designed performances or characteristics of the machine tool 30 is not expected.
FIGS. 8(A) and 8(B), which respectively correspond to FIGS. 7(A) and 7(B), schematically illustrate that the linear motor a has been assembled on the machine tool 30. When the linear motor 8 has been assembled, the armature core 18 is in close proximity to the permanent magnets 14, and as such, the magnetic attraction force imparted on the core 18 is very large, which is approximately several times the nominal (rated) driving force of the linear motor 8 (for example). As a result, the frictions between the guide rails 34a and 40a and also between the guide rails 34b and 40b are large to a considerable extent, and thus, such a large friction may result in decrease in the life time of the guides 34a-34b and 40a-40b. In order to overcome this problem, it is conceivable to increase the contact area between the linear guides, which, however, may arise another difficulties that the movable portion undesirably increases in weight and thus the acceleration of the movable body 12 is lowered.
One approach to overcoming the above-mentioned problems is disclosed in Japanese Laid-open Patent Application No. 10-257750, according to which a two opposing stators are provided between which a movable body is arranged to linearly move. In this case, the two magnetic attraction forces exerted on the two stators are cancelled, and thus, it is possible to reduce the friction between the liner guides of the stators and the movable body. However, the prior art disclosed in Japanese laid-open Patent Application is still encountered the problem that it is not easy to assemble the bear mortar. That is to say, according to this prior art, the two stators are firstly provided on the lower (bottom) frame of a machine tool as in the first prior art (FIG. 2(A)), after which the movable body, which has been fixed on the lower surface of a table, is lowered toward the space between the two stators. As mentioned above, the magnetic attraction force is very large and thus it is absolutely necessary to prepare a mechanism dedicated to the linear motor assembly as in the first prior art, which results in the same difficulty with the first prior art.
Therefore, in case the linear motor is assembled without use of such an assembly-assisting mechanism, it needs a quite long time until finishing the assembly on the machine tool due to strong magnetic attractions, and in this instance, it is usually not expected to precisely assemble the linear motor. Accordingly, it is often the case that the movable body is liable to be misaligned, resulting in the fact that the air gap between the stator and the opposing magnets is unable to be uniformly maintained. As a result, it is high liable to under induce cogging forces which lead to uneven driving forces.
It is therefore an object of the present invention to provide a linear motor which can be accurately assembled onto a machine tool in a short time period.
Another object of the present invention is to provide a linear motor which, once installed on a machine tool is able to considerably reduce the frictions between the engaged linear guides and also able to reduce cogging forces to a considerable extent.
One aspect of the present invention resides in a linear motor comprising; a stator having a magnet support which extends in parallel with the lengthwise direction of the linear motor and whose cross section perpendicular to the lengthwise direction of the linear motor is polygon or circular, the stator further having a plurality of permanent magnet arrays mounted on the outer surface of the magnet support in parallel with the lengthwise direction of the linear motor, the suitor being held at both ends thereof by stator supports; and a movable body having a hollow member extending in parallel with the length direction of the linear motor and surrounding part of the stator, and having a plurality of armature modules each of which comprises an armature core and armature coils mounted thereon, the plurality of armature modules being mounted on the inner surfaces of the hollow member such that the lengthwise axes thereof are in parallel with the lengthwise direction of the linear motor and such that the armature modules respectively face the permanent magnet arrays with an air gap therebetween.